Polymer latices particularly useful in coating composition, especially for coating of paper and board have to fulfill certain performance requirements. In order to adjust the performance properties of coating compositions comprising latices it is required to adjust the molecular weight and/or degree of crosslinking of the latex polymer by using chain transfer agents as regulator. One group of polymer latices used for paper coating applications in the prior art are obtained by emulsion polymerization of ethylenically unsaturated monomers comprising conjugated dienes, particularly 1,3-butadiene. Particularly for this type of polymer latices, due to the presence of a second ethylenically unsaturated group, crosslinking and molecular weight of the resultant latex polymer has to be carefully controlled in order to obtain the desired performance properties of the coating compositions.
The most well known group of chain transfer agents that are used as regulators to control molecular weight and degree of crosslinking of the polymer latices are sulfur-containing compounds, particularly mercaptans. The most prominent and very effective regulator used according to this concept is tertiary-dodecylmercaptan. The principle disadvantage of using sulfur-containing compounds, particularly mercaptan, in the emulsion polymerization of a polymer latex as chain transfer agent is the unpleasant odor of mercaptan compounds. It might already create labour safety issues during industrial production of the polymer latices. In particular in the area of packaging papers and packaging boards that are used for odor-sensitive applications sulfur-containing chain transfer agents should be avoided. Thus, in the past there have been efforts in the industry to substitute sulfur-containing chain transfer agents by other compounds that produce less odor, particularly for the preparation of polymer latices for paper coating applications.
One approach is disclosed in US 2004/0010079 wherein in the preparation of polymer latices the emulsion polymerization is conducted in presence of peroxo compounds as chain transfer agent. However, this approach has some disadvantages in that these compounds are rather expensive and the polymerization process has to be carefully controlled to avoid that the peroxo compounds used as chain transfer agent function as free radical initiators.
Another approach is known from WO 2008/017406 wherein a polymer latex with reduced odor is obtained by emulsion polymerization in presence of a chain transfer agent comprising a mercaptan having a linear alkyl group with 3 to 20 carbon atoms. Although it was found that by substituting the commonly used tert-dodecyl mercaptan by n-dodecyl mercaptan, the odor can be considerably reduced. Nevertheless, mercaptans are present in the obtained polymer latex.
But mercaptans have been proven to be very effective chain transfer agents to adjust crosslinking and molecular weight of polymer latices in order to obtain the desired properties. Thus, mercaptans are still commonly used in the industry as chain transfer agents.
As a consequence, there is still a need in the industry for polymer latices wherein the molecular weight can be appropriately adjusted for the desired end use whereby the addition of chain transfer agents and particularly mercaptan compounds can be reduced or totally avoided without compromising the desired product properties of the resultant polymer latex, particularly when used as binder in coating compositions for paper applications.
Furthermore, it is known from the prior art to use degraded starch in emulsion polymerization of ethylenically unsaturated monomers.
EP-A 536 597 discloses emulsion polymerization of ethylenically unsaturated monomers in presence of a degraded starch having a dextrose equivalent DE of 5 to 40. In the sole example in that patent application using butadiene, the commonly applied chain transfer agent tert-dodecyl mercaptan is employed in an amount of 3 wt.-%, based on the total amount of monomers. Furthermore, this explicit example uses a degraded starch having a DE of 17 to 19. There is no recognition within the entire disclosure of EP-A 536 597 that the presence of degraded starch and the specific selection of the DE of the degraded starch may have an influence on molecular weight control of the polymer latex in emulsion polymerization.
Furthermore, it is known from WO 2011/157679 to employ degraded starches in emulsion polymerization of ethylenically unsaturated monomers comprising vinyl-aromatic compounds and C1-C10-alkyl(meth)acrylates. But this reference neither discloses any effect of degraded starches on adjusting the molecular weight of the latex polymer nor that commonly used chain transfer agents can be reduced or avoided. On the contrary, this reference explicitly teaches to use sulfur-containing chain transfer agents in amounts of up to 5 wt.-%, based on the total weight of monomers.
In WO 2009/123637, degraded starches having a dextrose equivalent DE of 10 to 35 are disclosed as to have a synergistic effect if a latex polymerized in presence of these starches is used as binder in paper coating compositions comprising tetra-sulfonate-based fluorescence whitening agents. Again, there is no hint in this reference that by using degraded starches chain transfer agents can be reduced or avoided without compromising the properties of the resultant polymer latex.
U.S. Pat. No. 5,026,746 and U.S. patent application US 2011/0086567 disclose the use of starch-based binder prepared in presence of a degraded starch having a low DE equivalent of 1 to 20.
Thus, it is the object of the present invention to provide a low odor polymer latex that is useful as binder in coating compositions, particularly for paper and board applications without compromising the desired properties of the coating composition comprising such a polymer latex.